1. Field of the Invention
This invention relates generally to scanning probe microscopy and more specifically to scanning evanescent near field microwave and electromagnetic spectroscopy.
2. Description of the Related Art
Scanning probe type microscopes have typically been used to create visual images of a sample material. The image obtained may reflect any of a number of distinct electrical or magnetic properties of the sample material, depending on the parameter measured by the probe tip. For example, the tip may image electron tunneling, atomic force, absorption and refraction of propagating or evanescent electromagnetic waves, or other parameters. The tip may be in contact with the sample or it may be a short distance above the sample. A thorough discussion of scanning probe microscopes is presented by R. Wiesendanger, "Scanning Probe Microscopy and Spectroscopy: Methods and Applications" Cambridge University Press, 1994. Efforts in improving Scanning Probe Microscopes (SPMs) have focused almost entirely on increasing their resolution and sensitivity. While it is generally recognized that obtaining quantitative data to associate with the image detail would be highly desirable, two major technological barriers have prevented such instruments from being developed.
First, microscopy signals, as obtained from SPMs often are a combined function of topography and physical properties of the material. Separating them requires measuring at least two independent signals. For example, in scanning tunneling microscopy, the tunneling current is a function of both the tip to sample distance and the density of states. A recently developed scanning near-field optical microscope can measure optical signals such as luminescent spectra or optical index of refraction in addition to shear force, which can be used to determine the distance between tip and sample.
Second, to obtain quantitative information regarding the physical sample being imaged, complicated electromagnetic field equations in the region of the tip and sample must be solved. A review of this work is discussed by C. Girard and A. Dereux in Rep. Prog. Phys., vol. 657, 1996. Although numerical methods based on finite element analysis have been used to solve the field distribution around a near-field optical microscope tip, the complicated computational process involved, such as solving the Maxwell equations under real boundary conditions on a scale of a wavelength or less, is not practical in routine applications. The problem has been complicated for the work done in the past, because the microscopes were required to operate below a cut-off frequency and so suffered severely form waveguide decay, having a typical attenuation of 10.sup.-3 to 10.sup.-6 (R. F. Soohoo, J. Appl. Phys. 33:1276, 1962; E. A. Ash and G. Nichols, Nature, 237:510, 1972). In aperture or tapered waveguide probes, a linear improvement in resolution causes an exponential reduction in sensitivity. M. Fee, S. Chu, and T. W. Hansch, improved sensitivity and resolution to the micron level (Fee, M. et al., Optics Commun., 63:219, 1988) by using a transmission line probe with a reduced cross-section. However, further improvement in resolution was still accompanied by significant transmission line decay. The unshielded far-field wave propagation components around the tip of the transmission line probe significantly limited the resolution of the microscope, and particularly interfered with its use for quantitative analysis.
It would be highly desirable to have a scanning probe microscope capable of making images of features having submicron resolution and additionally capable of making quantitative measurements of the physical properties of the imaged features.